Low rank carbonaceous materials, such as brown coal, peat and lignite, are materials having water locked into a microporous carbonaceous structure. The water content is typically high—for example 60% or higher. This means that such materials have a low calorific value. Moreover, these materials have the undesirable mechanical properties of being soft, friable and of low density, meaning that they are difficult, messy and inconvenient to handle.
Prior processes for upgrading low rank carbonaceous materials (which for ease of discussion will be hereinafter collectively referred to as “brown coal”) have included “briquetting” and solar drying.
Briquetting typically involves heating the raw brown coal to remove excess water, then pressing the cooled brown coal into briquettes using an extrusion press or roll briquetting machine. However, briquetting is an expensive process due to the requirement for thermal energy and the mechanical wear on the extrusion press or roll briquetting machine.
The solar drying process involves milling of the brown coal with addition of water for long periods (e.g. up to 16 hours), then solar drying of the milled slurry in shallow ponds. This process is lengthy—particularly the solar drying step which may take up to several months—and energy intensive.
Another proposal mechanically releases water from brown coal by physically breaking up the coal. However, this process is inconvenient and time consuming and still requires lengthy air drying of the final product.
It is accordingly an object of the present invention to provide a process for upgrading brown coal which overcomes, or at least alleviates, one or more disadvantages of the prior art.